In Search of Past Underwater Light

by Marttiina Rantala

Importance of light

Have you ever been mesmerized by the beauty of light filtering through the water column, or the ripples of light dancing at the sandy bottom of a lake? While many can undoubtedly relate to such moments of awe, few have probably ever considered the implications of this play of light on the underlying biota or, indeed, on ourselves. In effect, light is essential for the underwater realm. As on land, light fuels life in lake ecosystems as photosynthetic organisms, such as algae, harness solar energy for their own growth, thereby making the precious energy available to all other lake organisms. Yet, sunlight also has a destructive side which most of us have very tangibly witnessed at some point in our lives. 

Play of light underwater.

The fundamental importance of light in aquatic systems is, however, much more than energy input for photosynthesis or the harmful effects of ultraviolet radiation. Notably, the energy from the Sun has a strong control over the movement of water in a lake basin. This movement, or lack of it for that matter, is not trivial as it governs the distribution and availability of resources, including light, nutrients and oxygen. Evidently, sunlight carries a host of consequences for aquatic life. What is more, there may be consequences that extend far beyond the boundaries of the lake itself. 

What controls the amount of light in lake ecosystems?

To meaningfully address the implications of underwater light, we need to understand processes governing light availability in lakes. The quantity and quality of light emitted from the Sun undergoes natural cyclic variations, though much of the variability is not controlled at the source. The location of a lake on the globe has a tremendous impact on the amount of light delivered: a polar or a mountain lake covered in ice may receive direct sunlight only for a couple of days a year while its counterpart in the tropics is being scorched relentlessly throughout the year. Intuitively, a change in the length of the ice cover period by even one day can have a substantial impact on lakes situated in the polar or mountain regions, denoting their vulnerability to environmental change [1, 2]. Similarly, the depth of a lake and any changes therein will have a marked influence on underwater light and consequently on aquatic biota [3]. 

There are further a number of substances in lake water that influence the amount of sunlight received by a lake ecosystem. Of particular importance is carbon, a small building block of life, which can absorb the bulk of sunlight when present in high abundance [4, 5]. A dark brown wetland lake or northern thermokarst lake (Figure 1) provide examples of aquatic systems brimming with light-absorbing carbon, being consequently depleted of light. Biological processes inside a lake may similarly affect the availability of light. In fact, the very algae fed by sunlight may begin to shade lower layers of the water column restricting algal growth at the bottom of a lake [6]. 

Figure 1. Thermokarst lakes in arctic Canada with highly variable carbon concentrations. Photo by Milla Rautio.

Local variability, global consequences

Evidently, variability in underwater light may be indicative of a number of global and local processes operating on time scales from minutes to millennia. All of these processes are intimately linked to elemental cycles in lakes, their surrounding land areas, and the atmosphere. All these processes are further prone to alter with ongoing climate change. In effect, a vast number of lakes are currently experiencing unprecedented changes in underwater light regimes, in many cases related to changes in the length of ice cover period or the input of light-absorbing carbon compounds from land [7, 8]. These alterations often have a marked impact on aquatic biota, yet they also carry broader implications that concern every living being on Earth. Lakes are an essential component in the global carbon cycle and take part in global climate regulation by acting as either sinks or sources of carbon [9]. Importantly, the observed changes in lakes hold potential for climate feedbacks, that is, processes that either amplify or curtail the effects of climate warming. The problem is we do not quite understand the mechanisms well enough to say which way the scale will tip globally: will lakes release or bind more carbon under the global change.

Tracing past variability in underwater light

To understand changes in the present or to make projections into the future, we need to take a look into past variability. Unfortunately, we do not have many records of past underwater light variability or related parameters. Then, how exactly do we go about capturing past variability in underwater light? This is the part where paleolimnologists begin to buzz with excitement. Indeed, lake sediments act as natural archives of environmental change storing information in their physical, chemical and biological features (read more) [10]. While we have no direct sedimentary gauge for underwater light, several variables provide indirect indications of past light variability. We may, for instance, take a look at the fossil remains of algae in sediments and look for species that are known to tolerate either shading or exposure to intense ultraviolet radiation. In addition to paleobiological indicators, sediment biogeochemistry can be used as a tool to trace variability in underwater light. 

One recently developed biogeochemical tool takes advantage of pigments synthesized by water fleas (Figure 2); a group of tiny macroinvertebrates with huge importance in lake ecosystems. Similar to human tanning, water fleas produce pigments to fight the damaging effects of ultraviolet radiation. These pigments are preserved in the tiny skeletons of deceased water fleas, which are preserved in lake sediments and subsequently form a record of past underwater light variability [11]. 

Figure 2. Water fleas with low pigmentation examined under a stereomicroscope. Photo by Marttiina Rantala.

I am currently involved in a project employing such bio-optical tools to unravel linkages between climate variability and aquatic carbon cycling in shallow polar and mountain lakes. Preliminary results from our experimental work strongly denote the influence of light not only on the complexion of water flea but also on a wide spectrum of biogeochemical parameters that may be stored in lake sediments. The next step will be to employ bio-optical tools on lake sedimentary records spanning several millennia, to decipher past changes in aquatic production, ice cover dynamics, and the interaction between lakes and the terrestrial environment, and to better understand the responses of lake ecosystems to natural and anthropogenic environmental change, as written in records of underwater light.

Marttiina Rantala, PhD
Postdoctoral fellow, Faculty of Geosciences and Environment, Institute of Earth Surface Dynamics, University of Lausanne

If you have questions or comments concerning Marttiina's post, please leave a comment below, or send her an email. You can also follow her research on ResearchGate.


[1] Prowse, T., Alfredsen, K., Beltaos, S., Bonsal, B.R., Bowden, W.B., Duguay, C.R., Korhola, A., McNamara, J., Vincent, W.F., Vuglinsky, V., Anthony, K.M.V. & Weyhenmeyer, G.A. (2011) Effects of changes in Arctic lake and river ice. AMBIO 40, 63–74.
[2] Griffiths, K., Michelutti, N., Sugar, M., Douglas, M.S.V., & Smol, J. P. (2017) Ice-cover is the principal driver of ecological change in High Arctic lakes and ponds. PLoS ONE, 12, e0172989.
[3] Vadeboncoeur, Y., Devlin, S.P., McIntyre, P.B. & Vander Zanden, M.J. (2014) Is there light after depth? Distribution of periphyton chlorophyll and productivity in lake littoral zones. Freshwater Science 33, 524–536.
[4] Solomon, C.T., Jones, S.E., Weidel, B.C., Buffam, I., Fork, M.L., Karlsson, J., Larsen, S., Lennon, J.T., Read, J.S., Sadro, S., Saros, J.E. (2015) Ecosystem consequences of changing inputs of terrestrial dissolved organic matter to lakes: current knowledge and future challenges. Ecosystems 18, 376–389.
[5] Karlsson, J., Byström, P., Ask, J., Ask, P., Persson, L., & Jansson, M. (2009) Light limitation of nutrient-poor lake ecosystems. Nature, 460, 506–509.
[6] Laurion, I., Ventura, M., Catalan, J., Psenner, R. & Sommaruga, R. (2000) Attenuation of ultraviolet radiation in mountain lakes: Factors controlling the among- and within-lake variability. Limnology and Oceanography 45, 1274 –1288.
[7] Smol, J.P., Wolfe, A.P., Birks, H.J.B., Douglas, M.S.V., Jones, V. J., Korhola, A., Pienitz R., Rühland K., Sorvari S., Antoniades D., Brooks S.J., Fallu M., Hughes M., Keatley B.E., Laing T.E., Michelutti N., Nazarova L., Nyman M., Paterson A.M., Perren B., Quinlan R., Rautio M., Saulnier-Talbot E., Siitonen S., Solovieva N & Weckström, J. (2005) Climate-driven regime shifts in the biological communities of arctic lakes. Proceedings of the National Academy of Sciences of the United States of America, 102, 4397–4402.
[8] Finstad, A.G., Andersen, T., Larsen, S., Tominaga, K., Blumentrath, S., de Wit, H.A., Tømmervik, H. & Hessen, D.O. (2016) From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes. Scientific Reports 6, 31944.
[9] Battin, T.J., Luyssaert, S., Kaplan, L.A., Aufdenkampe, A.K., Richter, A. & Tranvik, L.J. (2009) The boundless carbon cycle. Nature Geoscience 2, 598–600.
[10] Cohen, A.S. (2003) Paleolimnology: the history and evolution of lake systems. Oxford, UK, Oxford University Press.
[11] Nevalainen, L. & Rautio, M. (2014) Spectral absorbance of benthic cladoceran carapaces as a new method for inferring past UV exposure of aquatic biota. Quaternary Science Reviews 84, 109–115.


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